undefined | Better HN

0 pointskentonv12y ago0 comments

Hmm. The vtable approach used by FlatBuffer seems harmful for streaming. I don't think the SBE people would have used such a design.

> permitting new fields only at the end of a structure

Be careful not to conflate the order of fields as written in the schema with the order of fields on the wire. Cap'n Proto (like SBE) more or less requires new fields to go on the end on the wire, but lets you put them in any order in the schema. FlatBuffer apparently requires them to go on the end in the schema but lets you put them in any order on the wire.

(I say "more or less" because Cap'n Proto can actually stick a new field into what was previously padding space between two existing fields. But usually new fields go on the end.)

0 comments

2 comments · 1 top-level

jmillikin12y ago· 1 in thread

  > The vtable approach used by FlatBuffer seems harmful for
  > streaming. I don't think the SBE people would have used
  > such a design.

If the vtable and fixed-size fields are placed together with variable-length fields (e.g. byte blobs) at the end, then decoding can make better use of the processor cache than if the fields are all mixed together.

kentonvOP12y ago

But the idea with the vtable is that many instances of the same "table" (record) would share a vtable. So the vtable would presumably never appear inside the table instance, I think.

j / k navigate · click thread line to collapse